1. Field of the Invention
The present invention relates to a camera and an electroflash unit attachable to it.
2. Description of Prior Art
Various signals are transmitted from a camera to an electroflash unit attached thereto, and vice versa. The electroflash unit performs some functions in accordance with the signals transmitted thereto from the camera. The camera performs some functions in accordance with the signals transmitted thereto from the electroflash unit. However, electroflash units according to the prior art are unsatisfactory in transmission and reception of such signals for the following reasons:
In recent years, there has been widely employed an automatic flash output control system so-called TTL flash output control system for flashlight photographing with an electroflash device. In the known TTL flash output control system, the flashlight reflected by the film surface within the camera is received and measured by a photo-detector disposed within the camera. The current related to the received flashlight is integrated and the integrated voltage is compared with a reference voltage. The electroflash unit terminates flashlight emission at the moment when the integrated voltage and the reference voltage are in a predetermined relationship to each other.
The integrating circuit required for integrating the above light quantity and the reference voltage source for producing the above reference voltage may be provided on the camera or on the electroflash unit. However, at least the photo-detector and the circuit associated with the photo-detector are always provided on the camera according to the prior art. For the purpose of power saving it is preferable that the power supply to such circuit provided on the camera's side for TTL flash output control should be cut off excepting the time of flashlight photographing in TTL flash output control mode. However, if TTL flash output control circuit is normally kept in such a non-powered state, then a problem occurs in that when flashlight photographing is actually carried out, the photographer sometimes forgets to supply power to the circuit resulting in failure of the flashlight photographing.
On the other hand, the photographer may forget to turn the power source of the electroflash unit on after mounting the unit on the camera. In this case, he may fail to catch a good shutter chance or may erroneously carry out non-flashlight photographing for an object which should be taken with flashlight. In the latter case, the result is an improper exposure. Further, in case of the conventional electroflash unit, often the photographer may forget to turn the flash unit's power source off after the end of flashlight photographing and thereby the power source battery is consumed in vain.